1. Field of the Invention
The present invention relates to wiring boards and methods of producing the same.
2. Description of the Related Art
In organic packages arranged to employ ball grid arrays (hereinafter, abbreviated as BGAs), as a precondition, solder connecting portions acting as the BGAs are formed from solder balls (see, e.g., Japanese Patent Application Laid-Open Publication No. 10-112514 and “Development of Highly Reliable Sn—Ag Lead Free Solder” Toyota Central R&D and Labs., Inc. R&D Review, Vol. 35 No. 2 (2000), p39). Ordinarily, a solder connecting portion of a BGA is formed by placing a solder ball on a copper (Cu) pad formed on a board and using reflowing to form the connecting portion. Heretofore, a high temperature solder having a high Pb content has been used as a solder ball member. Since the high temperature solder has a high reflow temperature and is inferior in wettability with the Cu pad as compared with an eutectic solder, a faulty connection can occur in the high temperature solder (see, e.g., the above-mentioned patent and publication). Although the above-mentioned patent discloses stereotypical technology for improving wettability by applying a paste on the Cu pad, the result is not completely satisfactory. On the other hand, in another approach, a connection assistant solder layer is previously formed on the Cu pad by applying an eutectic solder paste on the Cu pad and then melting the paste. This approach can be expected to enhance the connecting strength of a solder ball by previously forming the eutectic solder layer having good wettability on the Cu pad.
More recently, so-called Pb free solder, i.e., a solder which does not contain Pb, is being used in place of the conventional Sn—Pb eutectic solder because of environmental pollution problems associated with the latter. The Pb free solder has a high reflow temperature and has poor wettability with the Cu pad. Accordingly, even if it is intended to form the connection assistant solder layer by melting, a problem arises in that the solder does not wettingly cover a pad surface well, and does not provide a good electrical connection because a large exposed area remains. Further, since quite a large amount of a free Pb solder swells high on the pad without wettingly covering the pad, when a solder ball is placed on the pad for mounting thereon, the ball becomes unstable on the pad. As a consequence, the ball positioning accuracy is poor. Moreover, when the amount of a solder paste to be applied is increased in an attempt to reduce the exposed area on the pad, the problem is exacerbated.